Pyridine-2,3-dicarboxylic acids are used as for example a raw material in the preparation of pharmaceuticals, agrochemicals and colorants. Pyridine-2,3-dicarboxylic acids can be prepared by various known processes. In these processes the product obtained usually is isolated by acidifying the reaction mixture and then crystallising and isolating it. The remaining mother liquor, which usually contains not only salts and by-products but also substantial amounts of pyridine-2,3-dicarboxylic acid, is difficult to work up. Recirculation causes an undesired accumulation of by-products, necessitating a relatively large blow-down with associated losses of pyridine-2,3-dicarboxylic acid. Furthermore, the stream is highly dilute.